events: introduce AercMsg and QueueRedraw

Add AercMsg as a main interface for internal communication in aerc in
preparation for a main event loop. Add a QueueRedraw function to to
trigger a redraw. This will be needed for widgets which should be drawn
after some delay (completions, terminal, for example)

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/ui/interfaces.go

@@ -4,6 +4,9 @@ import (
 	"github.com/gdamore/tcell/v2"
 )
 
+// AercMsg is used to communicate within aerc
+type AercMsg interface{}
+
 // Drawable is a UI component that can draw. Unless specified, all methods must
 // only be called from a single goroutine, the UI goroutine.
 type Drawable interface {

lib/ui/ui.go

@@ -7,6 +7,14 @@ import (
 	"github.com/gdamore/tcell/v2"
 )
 
+var MsgChannel = make(chan AercMsg, 50)
+
+// QueueRedraw sends a nil message into the MsgChannel. Nothing will handle this
+// message, but a redraw will occur if the UI is marked as invalid
+func QueueRedraw() {
+	MsgChannel <- nil
+}
+
 type UI struct {
 	Content DrawableInteractive
 	exit    atomic.Value // bool